Semiconductor devices using gallium oxide (Ga2O3) with a large band gap attract more attention as next generation switching devices achieving high voltage, low loss, and high heat resistance. Such semiconductor devices are expected to be applied to power semiconductor devices (power devices), such as inverters. According to NPL 1, gallium oxide has a band gap that may be controlled by forming mixed crystal with indium or aluminum, individually, or in combination of them. Among them, InAlGaO based semiconductors represented by Inx′Aly′Gaz′O3 (0≤X′≤2, 0≤Y′≤2, 0≤Z′2, X′+Y′+Z′=1.5 to 2.5) are extremely attractive materials.
PTL 1 describes a high crystalline conductive α-Ga2O3 thin film with a dopant (tetravalent tin) added thereto. The thin film described in PTL 1 is, however, not capable of maintaining sufficient withstand voltage and contains many carbon impurities, resulting in not yet satisfactory semiconductor properties including conductivity. It thus has been quite difficult to be used for a semiconductor device.
NPL 2 describes an α-Ga2O3 thin film may be formed on sapphire by MBE. It however describes that, although the crystal grows up to a film thickness of 100 nm at a temperature of 450° C. or less, the quality of the crystal becomes worse with a film thickness more than that and it is not possible to obtain a film with a film thickness of 1 μm or more.
An α-Ga2O3 thin film with a film thickness of 1 μm or more without crystal quality degradation has been, therefore, expected.
PTL 2 describes a method of producing an oxide crystal thin film by mist CVD using a bromide or an iodide of gallium or indium.
PTLs 3 to 5 describe multilayer structures having a semiconductor layer of a corundum crystal structure and an insulating film of a corundum crystal structure that are laminated on a base substrate of a corundum crystal structure.
PTLs 2 to 5 are publications on patents and a patent application by the present applicant.